This invention relates to a continuous vapor-phase process for the manufacture of alkyl mercaptans by reacting a dialkyl sulfide with hydrogen sulfide in the presence of a zeolite catalyst.
Alkyl mercaptans are produced commercially either by reacting an alkyl alcohol with hydrogen sulfide or by the addition of hydrogen sulfide to an alkene. In either case, the major by-product is the corresponding dialkyl sulfide, formed by the equilibrium reaction, EQU 2 RSH.revreaction.RSR+H.sub.2 S.
There are commercial applications for certain dialkyl sulfides, but in most cases the dialkyl sulfides obtained as by-products in mercaptan-manufacturing processes are contaminated with a number of other by-product impurities that make them difficult and uneconomical to purify, and they must, therefore, be disposed of as chemical waste.
One possible method to reduce the volume of the by-product sulfide waste and to recover economic value from it, is to react it with hydrogen sulfide, thereby reconverting the major dialkyl sulfide component of the waste to alkyl mercaptan according to the reverse of the above equilibrium reaction. Several procedures for cracking symmetrical dialkyl sulfides with hydrogen sulfide to form the corresponding alkyl mercaptan are described in the prior art. Beach, et al. (U.S. Pat. No. 2,667,515) provided a process for converting dimethyl sulfide and hydrogen sulfide to methyl mercaptan over a catalyst consisting of 10% cadmium sulfide on alumina at about 400.degree. C. Single-pass molar conversions to methyl mercaptan of about 59% were obtained. A Japanese patent (No. 77046203) discloses the use of a tungsten trisulfide on alumina catalyst at 320.degree.-450.degree. C.
Kubicek (U.S. Pat. No. 4,005,149) discloses the use of carbon disulfide, as a reaction promoter, in the presence of sulfactive catalysts such as cobalt molybdate on alumina to enhance conversions to alkyl mercaptan at lower reaction temperatures. Molar ratios of dialkyl sulfide/carbon disulfide ranging from about 0.1/1 to about 50/1 are employed and the process is shown in the examples to be operable for dialkyl sulfides ranging from dimethyl sulfide to di-n-dodecyl sulfide (dialkyl sulfides up to C.sub.40 are claimed). Catalyst temperatures in the range of about 400.degree. F. (204.degree. C.) to about 600.degree. F. (316.degree. C.) can be used. In U.S. Pat. No. 4,059,636, Kubicek further discloses the use of a supported phosphotungstic acid catalyst, a preferred embodiment being that carbon disulfide is also present in the reaction mixture to enhance the conversion of dialkyl sulfide to alkyl mercaptan at lower temperatures. Example 2 shows appreciably lower conversions, especially at the lower temperatures (204.degree.-288.degree. C.), when carbon disulfide is not used in conjunction with the phosphotungstic acid catalyst. Example 3 teaches that when dibutyl sulfide containing 24 mole % carbon disulfide is reacted with hydrogen sulfide over a phosphotungstic acid on alumina catalyst at 500.degree.-550.degree. F. (260.degree.-288.degree. C.), a single-pass molar conversion of di-n-butyl sulfide to n-butyl mercaptan of about 65% is obtained [i.e., percent conversion to C.sub.4 H.sub.9 SH is percent total conversion of (C.sub.4 H.sub.9).sub.2 S X % selectivity to C.sub.4 H.sub.9 SH/100].